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The number of ways of choosing with parameters
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At our disposal is a collection of $10$ red, $11$ blue and $12$ yellow fabrics. (each fabric is unique) In how many ways can we choose $4$ different fabrics if we want at least one fabric of each of the three colors?
My solution was since the first fabric chosen must be red, there are $10$ options for it. Then the next fabric must be blue, which has $11$ options. The third fabric is yellow, with $12$ options, and the last fabric can be any of the colors, provided that it has not already been chosen, so there are $(9+10+11-3)= 30$ ways to choose the last one, making the total number of choices $9cdot 10cdot 11cdot 30$.
My professor said that I needed to divide that by $2$ to get the right answer, but I just don't understand why. Any help would be much appreciated!
combinatorics discrete-mathematics
New contributor
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$begingroup$
At our disposal is a collection of $10$ red, $11$ blue and $12$ yellow fabrics. (each fabric is unique) In how many ways can we choose $4$ different fabrics if we want at least one fabric of each of the three colors?
My solution was since the first fabric chosen must be red, there are $10$ options for it. Then the next fabric must be blue, which has $11$ options. The third fabric is yellow, with $12$ options, and the last fabric can be any of the colors, provided that it has not already been chosen, so there are $(9+10+11-3)= 30$ ways to choose the last one, making the total number of choices $9cdot 10cdot 11cdot 30$.
My professor said that I needed to divide that by $2$ to get the right answer, but I just don't understand why. Any help would be much appreciated!
combinatorics discrete-mathematics
New contributor
$endgroup$
add a comment |
$begingroup$
At our disposal is a collection of $10$ red, $11$ blue and $12$ yellow fabrics. (each fabric is unique) In how many ways can we choose $4$ different fabrics if we want at least one fabric of each of the three colors?
My solution was since the first fabric chosen must be red, there are $10$ options for it. Then the next fabric must be blue, which has $11$ options. The third fabric is yellow, with $12$ options, and the last fabric can be any of the colors, provided that it has not already been chosen, so there are $(9+10+11-3)= 30$ ways to choose the last one, making the total number of choices $9cdot 10cdot 11cdot 30$.
My professor said that I needed to divide that by $2$ to get the right answer, but I just don't understand why. Any help would be much appreciated!
combinatorics discrete-mathematics
New contributor
$endgroup$
At our disposal is a collection of $10$ red, $11$ blue and $12$ yellow fabrics. (each fabric is unique) In how many ways can we choose $4$ different fabrics if we want at least one fabric of each of the three colors?
My solution was since the first fabric chosen must be red, there are $10$ options for it. Then the next fabric must be blue, which has $11$ options. The third fabric is yellow, with $12$ options, and the last fabric can be any of the colors, provided that it has not already been chosen, so there are $(9+10+11-3)= 30$ ways to choose the last one, making the total number of choices $9cdot 10cdot 11cdot 30$.
My professor said that I needed to divide that by $2$ to get the right answer, but I just don't understand why. Any help would be much appreciated!
combinatorics discrete-mathematics
combinatorics discrete-mathematics
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New contributor
edited 1 hour ago
Vinyl_coat_jawa
3,0101132
3,0101132
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asked 10 hours ago
cmplxlizcmplxliz
211
211
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$begingroup$
Because under your scheme you would count, for example, both
$$R1,B1,Y1,R2quadhbox{and}quad R2,B1,Y1,R1 .$$
But these are actually the same choice and therefore should not be counted twice.
$endgroup$
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$begingroup$
The total number of ways is $binom {10} {2} binom {11} {1} binom {12} {1} + binom {10} {1} binom {11} {2} binom {12} {1} + binom {10} {1} binom {11} {1} binom {12} {2} = 19800.$
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2 Answers
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$begingroup$
Because under your scheme you would count, for example, both
$$R1,B1,Y1,R2quadhbox{and}quad R2,B1,Y1,R1 .$$
But these are actually the same choice and therefore should not be counted twice.
$endgroup$
add a comment |
$begingroup$
Because under your scheme you would count, for example, both
$$R1,B1,Y1,R2quadhbox{and}quad R2,B1,Y1,R1 .$$
But these are actually the same choice and therefore should not be counted twice.
$endgroup$
add a comment |
$begingroup$
Because under your scheme you would count, for example, both
$$R1,B1,Y1,R2quadhbox{and}quad R2,B1,Y1,R1 .$$
But these are actually the same choice and therefore should not be counted twice.
$endgroup$
Because under your scheme you would count, for example, both
$$R1,B1,Y1,R2quadhbox{and}quad R2,B1,Y1,R1 .$$
But these are actually the same choice and therefore should not be counted twice.
answered 10 hours ago
DavidDavid
69.2k667130
69.2k667130
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$begingroup$
The total number of ways is $binom {10} {2} binom {11} {1} binom {12} {1} + binom {10} {1} binom {11} {2} binom {12} {1} + binom {10} {1} binom {11} {1} binom {12} {2} = 19800.$
$endgroup$
add a comment |
$begingroup$
The total number of ways is $binom {10} {2} binom {11} {1} binom {12} {1} + binom {10} {1} binom {11} {2} binom {12} {1} + binom {10} {1} binom {11} {1} binom {12} {2} = 19800.$
$endgroup$
add a comment |
$begingroup$
The total number of ways is $binom {10} {2} binom {11} {1} binom {12} {1} + binom {10} {1} binom {11} {2} binom {12} {1} + binom {10} {1} binom {11} {1} binom {12} {2} = 19800.$
$endgroup$
The total number of ways is $binom {10} {2} binom {11} {1} binom {12} {1} + binom {10} {1} binom {11} {2} binom {12} {1} + binom {10} {1} binom {11} {1} binom {12} {2} = 19800.$
answered 10 hours ago
Dbchatto67Dbchatto67
1,142118
1,142118
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cmplxliz is a new contributor. Be nice, and check out our Code of Conduct.
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